When does nginx return a Bad gateway (502)?

I have an application behind nginx (Rails on Unicorn if it matters) which listens on a UNIX socket.

It all works nice, especially when load is low. When there is some load on the server though (say 70%), I randomly(?) get a bunch of 502 responses -- usually in batches.

Thus the question: when (under what conditions) does nginx send a 502 response?

Thanks in advance,

fastcatch

PS: The app's worker processes do not seem to be irresponsive, slow, blocked for IO or anything else I have been able to identify as a possible cause. Thus I want to understand more the nginx side so as to find the root cause -- which I believe is on the other side (or some config tweak needed soemwhere).